Each of the documents listed below is referred to herein by the corresponding number enclosed in square brackets to the left of the document. Each of these documents is also incorporated herein by reference.    [1] Y. Liu, M. P. Fitz, and O. Y. Takeshita, “Qpsk space-time turbo codes,” in IEEE ICC, June 2000.    [2] X. Li and J. A. Ritcey, “Bit-interleaved coded modulation with iterative decoding, using soft feedback,” Electronic Letters, vol. 34, pp. 942–943, 4 Mar. 1998.    [3] X. Li and J. A. Ritcey, “Bit-interleaved coded modulation with iterative decoding,” in IEEE ICC, vol. 2, pp. 858–863, June 1999.    [4] X. Li and J. A. Ritcey, “Trellis-coded modulation with bit interleaving and iterative decoding,” IEEE Journal on Selected Areas in Communications, vol. 17, pp. 715–724, April 1999.    [5] X. Li and J. A. Ritcey, “Bit-interleaved coded modulation with iterative decoding,” IEEE Communications Letters, vol. 1, pp. 169–171, November 1997.
Coding and interleaving techniques are often used in wireless communication systems to improve the communication performance. FIG. 1 illustrates an example of a conventional wireless communication system described in [1]. This example implements turbo coding by using two convolutional coders (CC). One of the convolutional coders receives at its input the data stream that is to be transmitted, and the other convolutional coder receives at its input an interleaved (see 10) version of the data stream. The outputs of the convolutional coders are then modulated using QPSK (Quadrature Phase Shift Keying) and transmitted by respective transmit antennas. At the receiver, the signal from the antenna is input to a probability generator which generates symbol (or bit) probabilities. These symbol probabilities are fed to soft-input, soft-output (SISO) decoders that iterate to get estimates of the transmitted symbols (or bits). The SISO decoders use knowledge of the trellis of the convolutional coders to produce the estimates.
FIG. 2 illustrates an example of a conventional wireless communication system described in [2] and [3]. The system of FIG. 2 uses a single convolutional coder and an interleaver 21 before modulation and transmission by a single antenna. At the receiver, the signal from the antenna is demodulated and de-interleaved (see 22), and is then input to a SISO decoder. The a posteriori symbol probabilities output from the SISO decoder are interleaved (see 23) and fed back into the demodulator to get a better estimate of the symbol probabilities. This loop is iterated over. Systems similar to the one illustrated in FIG. 2 have also been suggested in [4] and [5], but those systems implement hard decoding decisions instead of soft decisions.
It is desirable in view of the foregoing to provide for improved performance in wireless communication systems that utilize turbo coding and transmit diversity.
According to the invention, in a wireless communication system with transmit diversity, a posteriori probabilities from SISO decoders are produced iteratively in alternating fashion to make symbol decisions. The SISO decoder associated with the weakest wireless communication channel goes first in the iterative process. This advantageously improves the symbol decisions.